A polarization plate for use in liquid crystal display devices is composed at least of a polarizer and two protection films opposing each other and sandwiching the polarizer. As the polarizer which composes this polarization plate, a film obtained by the following procedure is usually used. That is, a film of polyvinyl alcohol is prepared by a solution flow casting method, and the film is subjected to absorption treatment with iodine or a dichroic dye and then elongated in a boric acid solution.
As the protection film, a triacetylcellulose film (TAC film) is commonly used because of its excellent transparency. However, the TAC film has a high moisture permeability, and thus, its size can change due to moisture absorption under the high temperature and high humidity environment to cause an optical distortion. Therefore its reliability has not been always sufficient.
As an alternative protection film in place of the TAC film, there have been proposed films having low moisture permeability such as olefin based films or polyester based films. However, if the polarizer is attached to such a protection film having extremely low moisture permeability, removal of the moisture contained in the polarizer becomes insufficient and the moisture remains in the polarizer. This moisture disadvantageously reduces adhesiveness between the polarizer and the protection film.
In order to satisfy both moisture permeability and adhesiveness, there has been proposed a protection film obtained by laminating a cellulose ester based resin on the film having a low moisture permeability. For example, Patent Document 1 (JP 2004-226799-A) proposes to use a laminated film composed of a polyester resin layer and a hydrophobic cellulose ester resin layer as a protection film.
Patent Document 2 (JP 2002-331616-A) discloses a polarization plate comprising a polarizer and a laminate attached thereto as a polarization plate protection film, wherein the laminate is obtained by flow casting a resin composition composed of a maleimide/olefin copolymer and an acrylonitrile/styrene copolymer to form a film substrate, and then applying thereon a cellulose-based resin as an adhesive layer.
In addition, Patent Document 3 (JP 2001-215331-A) discloses a polarization plate obtained by attaching to a polarizer a laminate film, wherein the laminate film is obtained by applying the cellulose resin to a surface layer of a core layer composed of a resin layer.